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Full text of "The Cambridge Natural History"

CHAP,  in                        THE   THEROMORPHA                                pi
ground by mentioning the fact that among the Theromorpha
four distinct types of reptiles are included, which are considered
to form four orders, i.e. the Pareiasauri, the Theriodontia, the
Anomodontia (Dicynodontia), and the Placodontia.
The first of these divisions includes what seem to be basal
forms. These reptiles show numerous points of likeness to the
Amphibian Labyrinthodonts.1 On the other hand the third divi-
sion, that of the Dicynodontia, are highly - specialised Thero-
morpha, from which no further evolution would appear to have
been possible. Thus the dentition was either completely lost,
or reduced to tusks as in Dicynodon. We need not therefore
concern ourselves in the present volume with these Anomo-
donts. It is with the Theriodonts that our business lies.
The very name, be it observed, is aptly chosen on the hypo-
thesis to be explained here ; but it is not only in the teeth
that these reptiles show likenesses to the Theria or Mammals,
but in almost every feature of their organisation. Unlike other
reptiles, the Theromorpha in general were lifted comparatively
high above the ground on legs of fair length and of mam-
malian relationship in the position of the segments of the
limbs. The typical reptile grovels upon the earth with legs
sprawling out, as indeed the very name suggests. One bar to
the Theriodonts being on the direct line of mammalian ancestry
has been urged as a preliminary difficulty, and that is their
large size. The earliest undoubted mammals were small
creatures, comparable to a Hat or a Mouse in size ; whereas a good-
sized Bear or a Wolf is a better standard of size for some of the
best-known genera of Theriodonts. It has, however, been quite
permissibly suggested that living in company with these large
Theriodonts were less obtrusive genera, from which the mammals
might have sprung. It is so familiar a fact that a given group
of animals generally contains giants, dwarfs, and members of
intermediate size, that this suggestion may almost be accepted
as a fact. It need at least present no difficulties to us in our
comparisons.
The most salient " mammalian" feature of the Theriodonts
is the heterodonty of the teeth, the pattern of the '* molars/' and
the limited number which constitute the series. The fact, too,
that they are limited to the dentary bones below and to the
1 Of. vol. viii. p. 82.